1. Field of the Invention
The invention relates to pure transition alumina extrudates having good mechanical properties and a controlled pore size distribution which are suitable as catalyst supports and to methods of producing them. The extrudates have a bimodal pore size distribution with one pore volume of at least 0.10 cm..sup.3 /g. made of macro sized pores, a second pore volume of at least 0.60 cm..sup.3 /g. made of micro sized pores having a median pore diameter which is controlled in the range of 90-210 Angstrom units, and an intermediate pore volume of less than 0.05 cm..sup.3 /g. The extrudates can be further stabilized with a rare earth treatment.
2. Description of the Prior Art
Catalyst supports made of many different forms of alumina are known. However, little work has been reported directed to producing a pure transition alumina catalyst extrudate support wherein the pore size distribution is controlled to produce essentially a bimodal pore structure having three key properties. The first is to have a substantial pore volume made up of micropores in the range of 0-500 Angstrom units, which pores exhibit essentially all of the total surface area and which are narrowly distributed about the median pore diameter. The second is to have a significant volume of large macropores having diameters larger than 1,000 Angstrom units, but which exhibit only a very small fraction of the total surface area. The third is to have these extrudates exhibit excellent mechanical properties such as crush strength and attrition resistance in spite of their open pore structures. Nor has there been much discussion in the literature about this type of an extrudate where the large and small pore structures are interconnected by a very small fraction of pores of intermediate size in the range of 500-1,000 Angstrom units.
U.S. Pat. No. 3,898,322 to Leach discloses a catalyst support having a bimodal pore volume distribution with a large range of pore diameter distributions. The small size pores, which make up 20-40 percent of the total pore volume, consist of pores having a pore diameter from about 40 to 100 Angstrom units. The larger size pores have pore diameters from about 250 to 800 Anstrom units. Thus the overall pore size distribution tends to be toward smaller pores.
U.S. Pat. No. 3,770,617 to Riley et al discloses an alumina support having a specified pore size distribution with three pore size ranges identified. The largest size is made of pores of greater than 2,000 Angstrom units, and this makes up 3.0 to 10.0 percent of the total pore volume. The second size is made of pores ranging from 200-2,000, and this comprises a minor amount of 0.5 to less than 4 percent of the total pore volume. The last size identified consists of pores in the range of 30-80 Angstrom units which provide greater than 50 percent of the surface area. As seen in Examples 2 and 3 of the patent, about one-half of the total pore volume resides in these small pores and thus the Riley et al support can be characterized as having a small diameter micropore region. Furthermore, Riley et al requires the presence of 0.5-6 percent silica as a stabilizer, and thus there is no teaching of pure transition alumina.
U.S. Pat. No. 3,853,789 to Warthen et al relates to a macroporous alumina extrudate having a large surface area with a very substantial macroporosity. The amount of macroporosity is so great that over 40 percent of the pore volume is in pores greater than 3,500 Angstrom units. These extrudates do not have a substantial pore volume made of micropores having pore diameters in the range of 0-500 Angstrom units.
U.S. Pat. No. 3,898,155 to Wilson discloses an alumina catalyst for demetallizing and desulfurizing heavy oils. The catalyst has a Group VI-B metal and at least one Group VIII metal composited with alumina. This composite product has a macropore region made of pores greater than 600 Angstrom units comprising 10 to 40 percent of the total pore volume with the remainder of the pore volume in smaller micropores. The patent does not disclose how to produce the alumina support and so it provides little help to one wishing to produce an extrudate of this type. Neither does it provide any characteristics for the alumina support as a separate unit by itself. In the only two examples where an alumina support is utilized, there is no detailed characterization of the pore size distribution with regard to the nature of the distribution at various pore diameters. In both cases there is a relatively small amount of pore volume in the micropore region below 600 Angstrom units which is on the order of only about 0.527 cm..sup.3 /g., clearly below the limit of 0.6 cm..sup.3 /g. specified in the present invention.
There is another patent which discloses an extrudate that has a combination of large and small pore size distributions, although there is no focus on any criticality between the two sizes. This is U.S. Pat. No. 3,975,510 to Leach, which relates to a low density, high porosity alumina extrudate produced by treatment of the alumina with an organic solvent prior to the extrusion step. From the detailed pore size distributions of the micropore region below 500 Angstrom units given in Tables II and III of the patent, a relatively broad pore size distribution is seen about the median pore diameter which includes many small size pores. There is no achievement here of a narrow pore size distribution which is distributed about the median micropore diameter. The median micropore diameter itself is small and less than 88 Angstrom units. Furthermore there is no mention of the mechanical properties.